Automatic Vehicle Recognition (AVR) is a term applied to the detection and recognition of a vehicle by an electronic system. Exemplary uses for AVR include, for example, automatic tolling, traffic law enforcement, searching for vehicles associated with crimes, and facility access control. Ideal AVR systems are universal (i.e., they are able to read all license plates with 100% accuracy). The two main types of AVR systems in use today are (1) systems using RFID technology to read an RFID tag attached to a vehicle and (2) systems using a machine or device to read a machine-readable code attached to a vehicle.
One advantage of RFID systems is their high accuracy, which is achieved by virtue of error detection and correction information contained on the RFID tag. Using well known mathematical techniques (cyclic redundancy check, or CRC, for example), the probability that a read is accurate (or the inverse) can be determined. However, RFID systems have some disadvantages, including that not all vehicles include RFID tags. Also, existing unpowered “passive” RFID tag readers may have difficulty pinpointing the exact location of an object. Rather, they simply report the presence or absence of a tag in their field of sensitivity. Moreover, many RFID tag readers only operate at short range, function poorly in the presence of metal, and are blocked by interference when many tagged objects are present. Some of these problems can be overcome by using active RFID technology or similar methods. However, these techniques require expensive, power-consuming electronics and batteries, and they still may not determine position accurately when attached to dense or metallic objects.
Machine vision systems (often called Automated License Plate Readers or ALPR systems) use a machine or device to read a machine-readable code attached to a vehicle. In many embodiments, the machine readable code is attached to, printed on, or adjacent to a license plate. One exemplary ALPR system is shown schematically in FIG. 1, which illustrates the process of illuminating and viewing a retroreflective tag. The term “retroreflective” as used herein refers to the attribute of reflecting an obliquely incident light ray in a direction antiparallel to its incident direction, or nearly so, such that it returns to the light source or the immediate vicinity thereof. An infra-red light source 106 illuminates a retroreflective tag 102, which is located on a license plate 104. Retroreflective tag 102 reflects the infra-red light emitted by light source 106 straight back to the infra-red light source 106, where it is captured by an infra-red sensor 108, such as, for example, an infra-red camera. One advantage of ALPR systems is that they are can be used almost universally, since almost all areas of the world require that vehicles have license plates with visually identifiable information thereon. However, the task of recognizing visual tags can be complicated. For example, the read accuracy from an ALPR system is largely dependent on the quality of the captured image as assessed by the reader. Existing systems have difficulty distinguishing tags from complex backgrounds and handling variable lighting. Further, the accuracy of ALPR systems suffers when license plates are obscured or dirty.
Prior art methods of creating high contrast license plates for use in ALPR systems involve including materials that absorb in the infra-red wavelength range and transmit in the visible wavelength range. For example, U.S. Pat. No. 6,832,728 describes license plates including visible transmissive, infra-red opaque indicia. U.S. Pat. No. 7,387,393 describes license plates including infra-red blocking materials that create contrast on the license plate. U.S. Pat. No. 3,758,193 describes infra-red transmissive, visible absorptive materials for use on retroreflective sheeting.